Thunderstone Search Appliance Manual

To populate Additional Fields they should first be defined in the
Additional Fields section of All Walk Settings. You can specify
a name, which is used as the name of the XML element when displaying
the results, as well as when using the DataLoad API.

Once the field has been defined it can be populated either via the
DataLoad API or through the Data From Field settings section.
The fields are positionally numbered, and you can load Extra Field 1,
2 and/or 3 from the page that is read. If you are loading from a
<meta> field you will typically want a REX Search of
.+ and the From Meta Field you are loading from.